Transmissions of the roller transmission traction type are known. An example of this type of transmission is disclosed in U.S. Pat. No(s). 3,216,285, 3,254,546, 3,367,214, 3,364,761, and 3,417,370 to Nasvytis. This series of patents describe the basic roller transmission concepts relating to multiroller traction drives including improvements to vary the normal force applied to the rollers.
These types of transmissions theoretically have broad application in a variety of uses from marine to helicopter hybrid transmissions. Roller transmissions offer smooth, quiet, compact power transmission relative to geared transmissions, with high efficiency. In contrast to geared transmissions, which inherently generate significant torsional oscillations as the load transfers between teeth, roller transmissions are relatively vibration free. As such, the transmission is more reliable in high power, high speed applications where tooth growth and meshing become an increasing design problem.
A roller transmission transmits power through the traction forces on normally loaded rolling elements, utilizing a special traction fluid as both a lubricant and traction enhancer. The fluid used is typically a cycloaliphatic hydrocarbon which possesses elastohydrodynamic properties that allow it to essentially plasticize instantaneously while under the contact loads of the rollers. As such, torque is transmitted through the elastohydrodynamic film so that the rollers themselves are not in true contact. As the traction characteristic of the fluid is improved, ie, enhanced resistance to shear, the result is an increase in the maximum available traction coefficient with a consequent increase in power and speed capability at equivalent contact forces, ie, normal loads. This allows lower operational roller contact forces, reducing roller wear, heat generation and other power losses, and permits a greater maximum power and speed capacity. At all other times, this same fluid circulates throughout the transmission providing necessary lubrication and cooling.
In view of the critical role the tractive fluid plays in a traction drive, it is essential that the fluid be properly distributed, recycled, cooled, and redistributed to maintain the reliability, durability, efficiency, and functional integrity of the transmission.
To accomplish the placement of fluid at the roller interfaces, prior roller transmissions used an externally applied lubrication system with the rollers operating in a fluid bath. While this system functions at comparatively low rpm and low power systems, the fluid is not supplied in sufficient volume at high speeds to achieve required tractive and coolant effects, resulting in excessive heat build up. Slip or excessive wear, both of which may irreparably damage the roller surfaces, will then occur.
In view of the foregoing shortcomings of known roller transmissions to reliably transmit high power and torque at high speeds, the present invention proposes a lubrication supply and replacement system for adequately placing tractive fluid at the interface between adjacent rollers.
Another problem encountered in high speed roller transmissions is the physical expansion of component parts at high rotational velocities. The outer ring roller, in particular, because of its relatively large diameter and small radial section is subject to centrifugal growth and to changes in effective diameter with changes in load level. To accommodate these dimensional variations, the planet rollers in a transmission according to the present invention are equipped with movable bearings assemblies which accommodate limited roller growth.